[CostModel] Check for free intrinsics in BasicTTI

Recommitting part of "[CostModel] Unify Intrinsic Costs."
de71def3

Now that the 'free' intrinsic information has been sunk to the lowest
level, query the base implementation in BasicTTI before doing
anything else. I suspect this is the change that was causing the main
changes, particularly the large effects on debug builds.

Differential Revision: https://reviews.llvm.org/D80012